There is a significant demand for broadband mixers in present-day industry. These circuits are used in some reception and transmission circuits. It is possible to envisage numerous architectures for the production of mixers. These include, in particular, what are known as “balanced” or “double-balanced” structures which offer the best performance in terms of parasitic frequency rejection and very good channel isolation. There is a known way of producing a double-balanced mixer using a field-effect transistor bridge or a diode bridge. In order to produce such a circuit in MMIC (Monolithic Microwave Integrated Circuit) technology which can operate over a wide frequency range, it is necessary to use functional units known as broadband baluns.
The term “balun”, which is a contraction of the expression “balanced-unbalanced”, generally denotes a three-terminal device capable of either splitting an input signal into two output signals which have the same amplitude and have a phase difference of 180° between them, thus forming a power splitter with outputs in phase opposition (a splitter balun), or combining two input signals in such a way that the output signal represents the difference between the input signals, thus forming a combiner circuit with inputs in phase opposition (a combiner balun).
There are currently two known methods of producing baluns.
In the first method, shown in FIG. 1, mid-point transformers 12, 13 and 14 are produced and used to drive, for example, a field-effect transistor bridge 11. This device, which is passive and reciprocal, can advantageously provide both of the aforementioned functions. However, it is very difficult to use planar technology to produce a circuit of this type for operation at hyperfrequency. Moreover, it cannot be used for operation over a very wide frequency band (more than two to three octaves) or at low frequency (f<1 GHz).
In the second method, an active balun is used, based on a distributed structure, as described in the French patent application filed by the present applicant on 21 Dec. 2005 and published under the number 2 895 168. This active balun combines two balanced structures, one of which is composed of amplifier cells of the “cascade” or “Darlington” type, while the other is composed of amplifier cells of the “cascode” type. This specific circuit can be used, notably, to resolve the problem of gain unbalance between channels, which generally occurs in conventional circuits in the form of unbalance between the two output channels (in a splitter circuit) or between the two input channels (in a combiner circuit). It also enables a phase difference of 180° to be produced between channels.
This second method can be used to produce baluns operating at low frequencies and in much wider frequency bands (on the order of a decade or more) than those at which other existing devices operate. It is therefore a more advantageous solution than the first method. However, the active structures produced in this way are not reciprocal. Unfortunately, therefore, it is necessary to produce two different structures in order to produce a splitter and a combiner. If this type of structure is used in a mixer, the mixer will be unilateral.
Thus, regardless of which method is chosen, it is not possible at present to design an active balun, operating over a very wide frequency band (of the order of a decade), which is reciprocal and which can handle low frequencies (f≦1 GHz).
Consequently, anyone wishing to produce a reciprocal balun at the present time has to use a passive balun, for example a structure such as that of the Marchand balun. This structure, which is known and is illustrated in FIG. 2, is based on line coupling, with a main line 21 having a length of λ/2 and two lines 22 and 23 coupled to the main line and each having a length of λ/4, these two lines being arranged so as to provide signals of the same value and in phase opposition on their respective outputs 25 and 26. This structure can be used advantageously to produce reciprocal baluns. However, the use of passive couplers gives rise to problems of large overall dimensions, especially when operation at relatively low frequencies (around 1 GHz) is desired, since the passive coupler must always have a length of λ/2 with respect to the central operating frequency of the mixing device in which it is incorporated. This type of structure is therefore difficult to incorporate in MMIC technologies. Moreover, because of its structure, a balun of this type cannot be used for broadband applications (covering several octaves).